This application is generally related to a number of other applications which each define suspension control systems. Each of these related applications has been given an application number as listed below:
______________________________________ NAME APPLN. # FILING DATE ATTN-DOCKET # ______________________________________ Tsutsumi 07/520,185 May 8, 1990 78879CDCTY00 Tsutsumi 07/580,387 Sept 11, 1990 81282CDCTY Tsutsumi 07/580,388 Sept 11, 1990 81283CDCTY Tsutsumi 07/622,534 Dec 5, 1990 82667CDCTY ______________________________________
1. Field of the Invention
The present invention relates to a damping force control system for controlling the damping forces of shock absorbers depending on the acceleration of expansion of the shock absorbers.
2. Prior Art
One proposed damping force control system for controlling the damping forces of shock absorbers mounted on a motor vehicle is disclosed in Japanese Laid-Open Patent Publication No. 62-221907 published Sep. 30, 1987. The disclosed damping force control system detects road conditions based on the acceleration of expansion/contraction the shock absorbers, and varies the damping forces of the shock absorbers depending on the detected road conditions for thereby reducing vertical vibration of the motor vehicle.
Various devices have been employed to detect the acceleration of expansion/contraction a shock absorber. According to one arrangement, an expansion/contraction acceleration is detected based on a change in an output signal from a stroke sensor which detects the stroke of the piston in the shock absorber, as disclosed in the above publication. According to another proposal, a piezoelectric load sensor which comprises a laminated piezoelectric assembly is disposed between a shock absorber housing and a piston rod, and an expansion/contraction acceleration is detected based on a change in an electric charge generated by the piezoelectric load sensor.
The damping force control system includes a plurality of such expanding/contracting acceleration sensors attached respectively to the shock absorbers that are incorporated in suspensions by which road wheels are supported. The damping forces of the shock absorbers are independently controlled based on the signals detected by the respective sensors. If the detecting sensitivities of the sensors are different from each other, however, the damping forces of the shock absorbers tend to vary or switch over at different frequencies, resulting in poor riding comfort.
More specifically, the damping force of a shock absorber is varied as follows: The expansion/contraction acceleration of the shock absorber is detected by a stroke sensor or piezoelectric load sensor of the type described above. If the detected acceleration is greater than a preset threshold level, then the control system determines that the road surface has large irregularities, and reduces the damping force of the shock absorber. Therefore, if the sensors associated with the respective shock absorbers have sensitivity variations, then the damping forces of the shock absorbers are varied at different frequencies, and the motor vehicle gives passengers poor riding comfort.
To solve the above problem, it is necessary to employ expansion/contraction sensors having uniform sensitivity or output characteristics. However, it is practically difficult to make uniform the output characteristics of various sensors uniform. Even if the sensors are initially adjusted to uniform their output characteristics, their output characteristics will vary with time during usage.